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160 lines
6.0 KiB
Markdown
160 lines
6.0 KiB
Markdown
---
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toc_priority: 29
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toc_title: MaterializeMySQL
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---
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# MaterializeMySQL {#materialize-mysql}
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Creates ClickHouse database with all the tables existing in MySQL, and all the data in those tables.
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ClickHouse server works as MySQL replica. It reads binlog and performs DDL and DML queries.
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This feature is experimental.
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## Creating a Database {#creating-a-database}
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``` sql
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CREATE DATABASE [IF NOT EXISTS] db_name [ON CLUSTER cluster]
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ENGINE = MaterializeMySQL('host:port', ['database' | database], 'user', 'password') [SETTINGS ...]
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```
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**Engine Parameters**
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- `host:port` — MySQL server endpoint.
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- `database` — MySQL database name.
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- `user` — MySQL user.
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- `password` — User password.
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## Virtual columns {#virtual-columns}
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When working with the `MaterializeMySQL` database engine, [ReplacingMergeTree](../../engines/table-engines/mergetree-family/replacingmergetree.md) tables are used with virtual `_sign` and `_version` columns.
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- `_version` — Transaction counter. Type [UInt64](../../sql-reference/data-types/int-uint.md).
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- `_sign` — Deletion mark. Type [Int8](../../sql-reference/data-types/int-uint.md). Possible values:
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- `1` — Row is not deleted,
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- `-1` — Row is deleted.
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## Data Types Support {#data_types-support}
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| MySQL | ClickHouse |
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|-------------------------|--------------------------------------------------------------|
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| TINY | [Int8](../../sql-reference/data-types/int-uint.md) |
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| SHORT | [Int16](../../sql-reference/data-types/int-uint.md) |
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| INT24 | [Int32](../../sql-reference/data-types/int-uint.md) |
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| LONG | [UInt32](../../sql-reference/data-types/int-uint.md) |
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| LONGLONG | [UInt64](../../sql-reference/data-types/int-uint.md) |
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| FLOAT | [Float32](../../sql-reference/data-types/float.md) |
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| DOUBLE | [Float64](../../sql-reference/data-types/float.md) |
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| DECIMAL, NEWDECIMAL | [Decimal](../../sql-reference/data-types/decimal.md) |
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| DATE, NEWDATE | [Date](../../sql-reference/data-types/date.md) |
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| DATETIME, TIMESTAMP | [DateTime](../../sql-reference/data-types/datetime.md) |
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| DATETIME2, TIMESTAMP2 | [DateTime64](../../sql-reference/data-types/datetime64.md) |
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| STRING | [String](../../sql-reference/data-types/string.md) |
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| VARCHAR, VAR_STRING | [String](../../sql-reference/data-types/string.md) |
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| BLOB | [String](../../sql-reference/data-types/string.md) |
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Other types are not supported. If MySQL table contains a column of such type, ClickHouse throws exception "Unhandled data type" and stops replication.
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[Nullable](../../sql-reference/data-types/nullable.md) is supported.
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## Specifics and Recommendations {#specifics-and-recommendations}
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### DDL Queries {#ddl-queries}
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MySQL DDL queries are converted into the corresponding ClickHouse DDL queries ([ALTER](../../sql-reference/statements/alter/index.md), [CREATE](../../sql-reference/statements/create/index.md), [DROP](../../sql-reference/statements/drop.md), [RENAME](../../sql-reference/statements/rename.md)). If ClickHouse cannot parse some DDL query, the query is ignored.
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### Data Replication {#data-replication}
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`MaterializeMySQL` does not support direct `INSERT`, `DELETE` and `UPDATE` queries. However, they are supported in terms of data replication:
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- MySQL `INSERT` query is converted into `INSERT` with `_sign=1`.
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- MySQl `DELETE` query is converted into `INSERT` with `_sign=-1`.
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- MySQL `UPDATE` query is converted into `INSERT` with `_sign=-1` and `INSERT` with `_sign=1`.
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### Selecting from MaterializeMySQL Tables {#select}
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`SELECT` query from `MaterializeMySQL` tables has some specifics:
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- If `_version` is not specified in the `SELECT` query, [FINAL](../../sql-reference/statements/select/from.md#select-from-final) modifier is used. So only rows with `MAX(_version)` are selected.
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- If `_sign` is not specified in the `SELECT` query, `WHERE _sign=1` is used by default. So the deleted rows are not included into the result set.
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### Index Conversion {#index-conversion}
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MySQL `PRIMARY KEY` and `INDEX` clauses are converted into `ORDER BY` tuples in ClickHouse tables.
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ClickHouse has only one physical order, which is determined by `ORDER BY` clause. To create a new physical order, use [materialized views](../../sql-reference/statements/create/view.md#materialized).
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**Notes**
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- Rows with `_sign=-1` are not deleted physically from the tables.
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- Cascade `UPDATE/DELETE` queries are not supported by the `MaterializeMySQL` engine.
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- Replication can be easily broken.
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- Manual operations on database and tables are forbidden.
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## Examples of Use {#examples-of-use}
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Queries in MySQL:
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``` sql
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mysql> CREATE DATABASE db;
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mysql> CREATE TABLE db.test (a INT PRIMARY KEY, b INT);
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mysql> INSERT INTO db.test VALUES (1, 11), (2, 22);
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mysql> DELETE FROM db.test WHERE a=1;
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mysql> ALTER TABLE db.test ADD COLUMN c VARCHAR(16);
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mysql> UPDATE db.test SET c='Wow!', b=222;
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mysql> SELECT * FROM test;
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```
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```text
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+---+------+------+
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| a | b | c |
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+---+------+------+
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| 2 | 222 | Wow! |
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+---+------+------+
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```
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Database in ClickHouse, exchanging data with the MySQL server:
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The database and the table created:
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``` sql
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CREATE DATABASE mysql ENGINE = MaterializeMySQL('localhost:3306', 'db', 'user', '***');
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SHOW TABLES FROM mysql;
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```
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``` text
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┌─name─┐
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│ test │
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└──────┘
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```
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After inserting data:
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``` sql
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SELECT * FROM mysql.test;
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```
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``` text
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┌─a─┬──b─┐
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│ 1 │ 11 │
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│ 2 │ 22 │
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└───┴────┘
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```
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After deleting data, adding the column and updating:
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``` sql
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SELECT * FROM mysql.test;
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```
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``` text
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┌─a─┬───b─┬─c────┐
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│ 2 │ 222 │ Wow! │
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└───┴─────┴──────┘
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```
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[Original article](https://clickhouse.tech/docs/en/database_engines/materialize-mysql/) <!--hide-->
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